How Starfish Move — And The Water Vascular System

The underside of the starfish is covered with hundreds
of tube feet, which it uses for walking around, for attaching tightly
to rocks, and for holding on to prey. To move, each tube foot swings
like a leg, lifting up and swinging forward, then planting itself on
the ground and pushing back. At the tip of each tube foot (in most
species) is a suction cup. These aren't used when walking on level
ground, but can be used when walking up sheer surfaces.

With so many little legs, starfish can reach enormous
speeds — I've clocked Asterias vulgaris going over
30 centimeters per minute! (That's 60 feet per hour.) And some
species, like the inestimable sunflower star Pycnopodia
helianthoides, go even faster (Pycnopodia
gets up to four times as fast as Asterias).

Dermasterias imbricata
showing off its tube feet. This particular posture, with the tip of the
ray curled back and the tube feet extended, probably has more to do
with smelling than walking, and is very common.

Side view of one ray of a walking starfish, with tube
feet.
(From Brusca & Brusca, Invertebrates.)

A single tube foot in motion.
(Adapted from Brusca & Brusca.)

The bulb at the top of the tube foot (it's actually inside the
starfish, but it's easier to see this way) is called the ampulla.
When the ampulla contracts, it's squeezing water down into the tube
foot, which then extends. To retract the tube foot, the ampulla
relaxes. So, extension and retraction of the tube feet are by water
pressure. (But muscles are used to bend the tube feet.) The tube feet
and the ampullae are part of the water vascular system
of the starfish, which connects them all together.

Here's what the water vascular system would look like if the
rest of the starfish wasn't there (thanks to Brusca & Brusca
again):

The ring in the center is the Ring
Canal. It's in the central part of the starfish. A Radial Canal runs
down the length of each ray, with a row of ampullae and tube feet on
each side of it. Also coming off the Ring Canal is the madreporite. The
madreporite is like a sieve or strainer, and filters out particles in
the water. It allows the water to enter the water vascular system from
outside the starfish.